At the heart of many of New York’s tallest residential skyscrapers lie “mechanical voids.” They are growing in size and number and the city council and Mayor Bill de Blasio have had enough.

From outside, they are often just windowless strips covering the equivalent of multiple floors. Inside is equipment to keep the building running, and empty space, sometimes a lot. Height added by mechanical rooms doesn’t count toward a tower’s floor area that developers must get approved by the city, so they can be used to make buildings taller. With greater height comes more expensive penthouse views and the bragging rights that come with stature.

Critics see the practice as potentially dangerous, forcing firefighters or medical workers responding to an emergency to cover much greater distances—sometimes unanticipated, since the voids aren’t numbered floors.

“It is a symptom of everything that is becoming wrong with our society that developers would rather build empty spaces in buildings for billionaires than affordable housing,” said New York City Council member Ben Kallos. “We’re not saying that you can’t do this. What we’re saying is that if you have a limited amount of floor area that you can use to put up a building, then you have to use that floor area.”

Loophole closing

An amendment filed by the planning department at de Blasio’s request would limit mechanical spaces to a height of 25 feet, and require multiple mechanical floors to be at least 75 feet apart. Otherwise, they would count toward the building’s floor area as set by zoning rules, which determine how tall a building can be.

Kallos worries that without intervention, the mechanical voids will just keep growing—to 300 or 400 or 500 vertical feet of dead space. The practice is especially noticeable on Billionaire’s Row—a strip of super-luxury condo buildings just south of Central Park. Mechanical voids make up about a quarter of 432 Park Ave., Manhattan’s tallest completed condominium tower, according to Kallos. The building’s minimalist boxy design can be seen from every borough.

Not waiting

Even ahead of an official rule change, the city’s Department of Buildings said it notified developers of two proposed Manhattan residential towers that it would revoke permits or stop construction because of concerns the buildings were potentially unsafe or the space was being inappropriately used.

Although construction on the foundation is allowed to continue on a Rafael Viñoly-designed building slated for the Upper East Side, the developers need approval from the fire department for emergency access plans for outdoor space slated for the middle of the building, which would boost the apartment atop it higher, if they want to start work on the tower itself, said Andrew Rudansky, a spokesman for the department. Public documents show the building would reach 510 feet, but have only 32 floors and 83 units. Rafael Vinoly Architects declined to comment. Real Estate Inverlad, a developer on the project, didn’t return requests for comment.

On the other side of Central Park, the city threatened to revoke previously approved permits for Extell Development’s planned 775 foot condominium tower near Lincoln Center, Rudansky said in an email. The builders indicated they would resolve the department’s objections in order to move forward with the 40-plus story tower. The 18th floor, which would hold mechanical equipment, was expected to take up nearly 160 vertical feet. Extell didn’t respond to requests for comment.

Its estimated completion date is still slated for 2021, according to the company’s website, with 127 condos, a squash court, indoor and outdoor pools and bowling alley. Permits for a new 25-story building are currently approved for the site.

Wiggle room

The city’s amendment is a move in the right direction, but leaves too much wiggle room, Manhattan Borough President Gale Brewer said during a public meeting. It doesn’t address unenclosed spaces such as terraces, and would affect only parts of the borough, leaving out specific parts of Billionaire’s Row, which she said “is facing an imminent threat.” The proposed zoning changes would cover areas on the Upper East and West sides, Midtown and near Manhattan’s southern tip.

“We need a more comprehensive approach,” she told the City Planning Commission. “The point of closing a loophole is that you do not leave an opening that can be exploited by developers.”